Flavobacterium columnare, a causative agent of columnaris disease, causes great annual economic losses for the cultured catfish industry in the United States and many other cultured fish species [1]. It has been recognized as a world-wide pathogen of freshwater fishes affecting wild, cultured and ornamental fish populations including the commercially important species [2-4]. Epizootics of columnaris disease frequently occur in natural populations and have been devastating/troublesome in fish farms, hatcheries, ponds, pens and cages particularly in fishes held under intensive culture conditions with poor water quality [5]. In the catfish industry, columnaris disease is the leading cause of catfish mortality, in-pond mortality rates among adults and fingerlings can reach 60 and 90%, respectively [6]. Columnaris is one of the most frequently occurring disease in catfish, representing an average of 41% of the total cases submitted for diagnosis from 2001 to 2010 [7]. Columnaris disease or mixed infections including columnaris is considered the greatest cause of economic loss on catfish farms by more than 70% of the catfish farmers polled in the four leading catfish-producing states [8]. Catfish at any age, during all seasons, and under variety of water conditions are susceptible to columnaris infection [9].
The intensification of aquaculture operations has increased the incidence of infectious diseases [10]. Sustainable development of aquaculture relies on disease prevention and vaccination has become the best tool to achieve that goal. Effective vaccines are ultimately the safest prophylactic approach to evade infectious diseases. Moreover, the use of antibiotics in aquaculture has been reduced dramatically since the introduction of vaccines [11].
F. columnare is a genetically heterogeneous group of pathogens, divided into three main different genomovars and genetically different strains within the species demonstrate various levels of virulence for different fish species [12-15]. Genomovar II strains were found to be significantly more virulent to channel catfish than genomovar I strains and have higher adhesion capacity as well [14, 16]. Rifampicin resistance strategy has been successfully used to develop modified live attenuated bacterial vaccines for commercial use in aquaculture [17, 18]. Currently, a modified live F. columnare vaccine is available for commercial use to prevent columnaris disease under the licensed name AQUAVAC-COL™. The active ingredient in this vaccine is an avirulent rifampicin-resistant mutant of F. columnare genomovar I, the less virulent group. The efficacy of this vaccine has been publically questioned from farm settings. Recently, inventors have developed safe, and permanently stable, rifampicin-resistant mutants from genomovar II strains, the highly virulent group. Since genomovar II strains are more virulent, a specific vaccine targeting this genomovar is likely to increase the protective effect of vaccination.
Therefore, the purpose of this study was to evaluate the efficacy in zebrafish, channel catfish, and Nile tilapia of the new modified live F. columnare vaccines (mutants) developed by repeated passage of virulent genomovar II strains on a medium containing increasing concentrations of rifampicin. It is thus a primary object of the instant disclosure to provide a safe and effective F. columnare vaccine.